So - unlike most everyone else I don't think it is (yet) cut and dried, because your issue now will be proving it. Or at least one issue is proving it.
Your position would be stronger if you knew (a) whether anyone else who had not had a full 12 months in work (for any reason) was also downgraded, and (b) what reasons other people downgraded were given. Of course the problem here is that you aren't entitled to that information.If the pool of people were bigger, then you could require the information in an anonymised form - but given the small numbers I doubt if anonymising the information would work.
Your second problem is the one that I raised earlier - how far are you willing to take this, and what are the consequences? There is no magic employment fairy who solves your problems for you. I know you know that, although I do sometimes despair of people who obviously think there is. If you wish to challenge this your first action must be to immediately raise a grievance and see it through to the bitter end. The employer might re-think if you do that - but if they do it doesn't mean they will like it, nor that they can't do anything about not liking it. You may find that any promotion you have your eye on never works out. Again, not saying they would take it out on you or hold a grudge forever - but only you know how your employer is likely to react.
Once you have exhausted (and lost) the whole grievance procedure including the appeal, then you can lodge an employment tribunal claim for discrimination. You need to keep an eye on the clock - you only have three months less a day from the date of the action you are complaining about, so if the grievance is delayed you may have to claim before it is completed.
There are massive delays in tribunals now, and discrimination are more complex cases, so you may be looking at anything between 12 and 24 months before your case comes up, whilst having to work with your employer.
Or you can resign NOW and claim constructive dismissal and discrimination which I won't go into detail about right now because I would never advise anyone to go down this route unless they are deperate (and possibly foolish) - the chance of winning are almost nil.
In either of these cases you may end up with a settlement. Although it is likely that not having a job would be part of any deal - again that may not be the case, but it more often than not is.
The better part of valour is to suck it up - again I am not recommending that you do, simply outlining your options. In terms of risk to your employment, this carries the least risk. Worst case scenario is that you get less bonus this year - and again, whilst I am not convinced entirely by the argument, you might choose to see this as a reasonably "fair" outcome overall if others have had to work the full 12 months to get the same level of bonus. Best case scenario is that you don't get as much bonus but your acceptance marks you as a "team player" and increases your stock with the employer, which long term might be a good thing - again, not saying it will be the case, just that it's a possible option.
I think that covers most potential scenarios. I wouldn't recommend any one over another because that isn't my decision to take. There are potentially benefits or disbenefits to all the options, so whatever you chose has to be what you are willing to live with and based on how you think your employer would respond.